A. Field of the Invention
This invention relates to microwave mixers and more particularly to coplanar strip line mixers.
B. Background Art
It is well known in the art that the basic circuit of a mixer, or frequency converter, consists of a microwave structure which combines two signals and feeds them into at least one mixer diode whose nonlinear characteristics provide mixing or frequency conversion. The frequency conversion involves the use of the nonlinear properties of the diodes to create difference-frequency (IF) voltages which are then amplified. Specifically, a local oscillator provides an inphase signal (LO) to the mixer. A radio carrier signal at a particular frequency (RF) is also provided to the mixer diode configuration. The mixer modulates the radio carrier signal amplitude with the LO signal thereby forming a modulated carrier signal having difference frequencies or intermediate frequencies (IF) i.e. the difference between RF frequency and LO frequency. The intermediate frequency signals have the same phase and can be combined together to provide the composite output. The mixer essentially acts as an IF amplifier. The current through the mixer diodes configuration is a function of the sum of the LO signal and the RF signal.
An important feature of balanced mixers is the isolation provided between the RF signal input port and the LO signal input port. If the RF signal came from a receiving antenna, the isolation would prevent energy provided at the LO input port from being transmitted to the antenna and possibly radiating into space. Since it is a very difficult task to match the mixer diodes perfectly at all frequencies and under all LO drive conditions, the isolation between the RF input port and LO input port is a function of the return loss of the diodes.
Generally, desirable characteristics of a balanced mixer include amplitude modulated (AM) noise cancellation, LO to RF isolation and simplicity of construction. A double balanced mixer may be constructed using toroidal transformers or strip line techniques. There are several types of doubly balanced microwave mixers including the balun-ring, disclosed by Neuf in U.S. Pat. No. 3,652,941, and star configurations. However, the type most frequently used for strip line construction is the pseudo double balanced mixer. This consists of two conventional balanced mixers built using either 90 degree or 180 degree hybrids placed between two isolating 90 degree hybrids. Thus, the RF signal splits to both balanced mixers as does the local oscillator input signal. Because of the circuit configuration, the LO and RF signal leakages through the balanced mixers are shunted to the isolating hybrids. Similarly, the reflections from the two balanced mixers are shunted into the hybrid loads, thus providing a mixer which has good LO to RF signal isolation and good voltage standing wave ratio (VSWR) at all ports. Also see Gupta, Stripline Circuit Design, pages 269-291 incorporated herein by reference.
The output IF signal bandwidths is a function of both the RF frequency of the mixer as well as its doubly balanced configuration. Strip line mixers are commonly built with IF frequencies and bandwidths going up to several hundred MHz, with 500 MHz being a reasonable upper limit for conventional design.
Balanced mixers have LO to RF isolations which are a function of their circuit types. However, doubly balanced mixers have an inherently higher degree of LO to RF isolation as a result of their dual hybrid construction. Typically, minimum levels of isolation equal to 20 dB can be anticipated, with even higher isolation over narrow frequency ranges. Furthermore, as in the case of isolation, the doubly balanced mixer solves the problem of reduced VSWR. Doubly balanced mixers can be typically expected to operate at VSWR's less than 1.5, i.e. providing large reflection with a high degree of isolation.
There are three basic diode types which are applicable to a mixer design. These are the point-contact diode, the back diode (a variation of the tunnel diode) and the Schottky diode. Point-contact diodes have high RF impedances which generally makes their matching somewhat difficult over broad bandwidths. Back diodes have low RF impedances which makes them readily matched over a broad frequency range. The impedances of Schottky diodes is more dependent upon LO drive level than is the impedance of either the back diode or the point-contact diode. Thus, with proper LO drive, it is possible to match a Schottky diode over an extremely broad bandwidth, up to and including 20:1. Also, application of a dc bias improves the match of a Schottky device over a broad band width, particularly in the higher frequency ranges. At high frequencies, the parasitic inductance values of the diode configurations cannot be ignored since they limit the isolation to 20dB and less for a single diode mounted in a 50 ohm line.
Baluns have been used as the coupling device for connecting local oscillator signals to mixer circuits. A balun is the common name for an unbalanced-to-balanced converter. Baluns have been constructed for microwave applications using either waveguide or coaxial line techniques. The function of the balun is to convert an unbalanced grounded signal into a balanced ungrounded signal for circuits operating in a ground isolation mode. Although compensated coaxial line and waveguide-type baluns have been developed, their use is limited because of their size and unfavorable electrical characteristics.
In a strip line balanced mixer circuit, a coupling device must also be provided for connecting the local oscillator signal to the mixer circuit. Coaxial lines or waveguide-type baluns cannot be used because of their incompatibility with strip line design. Previously the coupling device used was a strip line coupler circuit consisting of several quarter wavelength strip lines which could be larger than the mixer itself. However, one of the benefits of strip line circuit elements are their small size. Therefore, a coupler having strip lines larger than the mixer itself would not be acceptable for many applications.
When attempting to reduce the size of a balun-mixer fully coplanar strip line circuit, structural, topological problems become apparent.
Specifically, in designing a fully coplanar strip line analogy to a coaxial line circuit, it is not necessary to physically connect the strip lines to the mixer diodes if the strip lines are curved to within a predetermined special distance from the mixer diode configuration. However, by curving the strip lines to conform to a coaxial line configuration, the analogous characteristics of a coaxial line mixer circuit no longer exist because one strip line representing the inner conductor of a coaxial line will be shorter than the other strip line that represents the coaxial outer conductor. It, therefore, becomes necessary to make compensation in gap spacing between strip lines and to optimize the radius of curvature of the strip lines. Otherwise, if the strip lines were not curved, a straight section of coplanar strip line would have to be connected to the diode configuration by an indeterminate length of wire that would be outside the plane of the striplines.
Therefore, an object of this invention is a fully coplanar strip line mixer circuit coupled to a balun that provides optimal isolation between the LO input port of the balun and the RF input port of the mixer.
Another object of this invention is a coplanar strip line circuit having an impedance that matches the impedance of any other device used with it as a function of the mixer diode characteristic.